proto.h: Use __attribute__nonnull__ if available

proto.h contains a generated PERL_ARGS_ASSERT macro for every function.
Prior to this commit, each such macro will assert that each such parameter
isn't NULL.  Some compilers allow a compile-time assertion to be made
for this situation.  This commit uses that instead when available, but
only for functions that don't have a thread context.

The reason for that is it gets more complicated; so I'm starting with
this.

Author: khwilliamson